AVS2018 Session TF-TuM: Emerging Applications for ALD

Tuesday, October 23, 2018 8:00 AM in Room 101A
Tuesday Morning

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8:00 AM TF-TuM-1 Atomic Layer Deposition of the Metal Pyrites FeS2, CoS2, and NiS2
Xinwei Wang (Shenzhen Graduate School, Peking University, China)

The pyrite-type transition-metal disulfides (MS2, M = Fe, Co, Ni) form a series of compounds that are highly interesting in many aspects. These compounds share the same cubic pyrite crystal structure but differ in the progressive increase of an anti-bonding d electron in the conduction band, and as a result, the metal pyrites exhibit very diverse and intriguing electrical magnetic properties from diamagnetic semiconductive to itinerant-electron ferromagnetic and to antiferromagnetic semiconductive. This diversity of the material properties has not only offered a model system platform for fundamental science studies but also enabled tremendous engineering possibilities for practical applications, such as solar cells, lithium/sodium-ion batteries, and electrocatalytic hydrogen evolution, oxygen evolution, and oxygen reduction.

In this presentation, we will show our latest progress on the development of atomic layer deposition processes for the metal pyrites of FeS2, CoS2, and NiS2 (Angew. Chem.Int. Ed. 2018, doi:10.1002/anie.201803092). We use the metal amidinate compounds as the precursors for the metals and H2S plasma as the sulfur source, and we will show that the deposition processes for FeS2, CoS2, and NiS2 all follow ideal layer-by-layer ALD growth behavior over a wide temperature range to produce fairly pure, smooth, pyrite-structure metal disulfide films. We will further show that the ALD FeS2, CoS2, and NiS2 films can be conformally deposited into deep narrow trenches with aspect ratios as high as 10:1, which thereby highlights the broad and promising applicability of these ALD processes for conformal film coatings on complex high-aspect-ratio 3D architectures in general.

8:20 AM TF-TuM-2 Atomic Layer Deposition of Yttrium Fluoride and Yttrium Oxyfluoride Films with Tunable Stoichiometry
Jasmine Wallas, Jessica Murdzek, Diane K. Lancaster, Andrew Cavanagh, Steven George (University of Colorado at Boulder)

YF3 and YOxFy are materials with excellent chemical and thermal stability. YF3 and YOxFy have both demonstrated exceptional corrosion resistance to highly reactive plasmas. In this work, YF3 atomic layer deposition (ALD) was developed using tris(butylcyclopentadienyl) yttrium and HF-pyridine as the reactants. The ALD of YOxFy alloys was also demonstrated with tunable control of the oxygen and fluorine stoichiometry. This tunable control was difficult because of the rapid exchange of oxygen by fluorine in Y2O3 and YOxFy alloys during HF exposures.

In situ quartz crystal microbalance (QCM) analysis of YF3 ALD revealed linear mass changes and self-limiting behavior using tris(butylcyclopentadienyl) yttrium and HF-pyridine as the reactants. The mass gain per cycle (MGPC) was 21.5 ng cm-2 at 225°C. The growth rate of YF3 ALD was also determined to be 0.3 Å per cycle by ex situ X-ray reflectivity analysis. Energy dispersive spectroscopy (EDS) of a cross-section of the YF3 film yielded a uniform 3:1 ratio of F:Y with low impurities.

YOxFy alloys were deposited using H2O together with the tris(butylcyclopentadienyl) yttrium and HF-pyridine reactants. However, control of the composition of the YOxFy alloys was complicated by the facile exchange of oxygen by fluorine during the HF exposures. The oxygen/fluorine exchange was most obvious during HF exposures on Y2O3 ALD films when a large mass gain was observed during fluorination. The Y2O3 fluorination reaction is believed to be: Y2O3 + 6 HF → 2 YF3 + 3 H2O. In addition to the large mass gain, the presence of fluorine throughout the entire film was revealed by X-ray photoelectron spectroscopy (XPS) measurements with depth-profiling. The XPS depth-profiling results are consistent with rapid fluorine diffusion in the Y2O3 and YOxFy films.

Various super-cycles were employed to obtain YOxFy alloys with particular F:O ratios. The most reliable method for composition control was defined by performing HF exposures between intervals of Y2O3 ALD. The number of Y2O3 ALD cycles and the length of the HF exposure could be varied to produce YOxFy alloy films with tunable and consistent composition as measured with XPS depth-profiling. The growth rate of the YOxFy alloy films was dependent on the number of Y2O3 ALD cycles before the HF exposures. The super-cycles with a larger number of Y2O3 ALD cycles before the HF exposures produced higher growth rates resulting from the higher growth rate of 0.8 Å per cycle for Y2O3 ALD.

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8:40 AM TF-TuM-3 Synthesis of Single Phase Two-dimensional SnS2 by Plasma-enhanced Atomic Layer Deposition
JungJoon Pyeon, In-Hwan Baek (Korea Institute of Science and Technology); Taek-Mo Chung (Korea Research Institute of Chemical Technology); JeongHwan Han (Seoul National University of Science and Technology); Chong-Yun Kang, SeongKeun Kim (Korea Institute of Science and Technology, Republic of Korea)

Two-dimensional (2-D) metal chalcogenides have received great attention because of their unique characteristics. A challenge in implementing 2-D metal chalcogenides in emerging devices is to synthesize a well-crystallized layer over large areas at temperatures compatible with current fabrication processes. Tin disulfide, a n-type layered semiconductor, is a promising candidate for realizing large-area growth at low temperatures because of its low melting point. However, tin sulfides exist in various phases such as SnS, Sn2S3, and SnS2. It is challenging to form a single phase SnS2 at low temperatures.

Here, we demonstrated the synthesis of high-quality SnS2 by plasma-enhanced atomic layer deposition (PEALD). All the processes were performed below 300 oC, which is compatible with current electronic devices. Stoichiometric SnS2 films were formed. It was verified from various techniques such as Raman spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffraction that a single phase of SnS2 was formed. It was found that the microstructure of the SnS2 films was strongly dependent on the growth temperature. Vertically aligned SnS2 flakes were observed in the SnS2 grown at relatively high temperatures (210 - 270 oC). Through a two-step process – a seed layer growth at low temperature (150 oC) and a main layer growth at high temperatures (240 – 270 oC), well crystallized SnS2 layers were aligned in parallel to the substrate. Moreover, the properties of field-effect transistor using the SnS2 films were investigated. A high on/off ratio of ~ 106 and a moderate field-effect mobility of ~ 1 cm2/Vs were achieved from the devices utilizing SnS2 grown by PEALD.

9:00 AM TF-TuM-4 Phase Selective, Low Temperature Growth of TiO2 by Atomic Layer Epitaxy
Jason Avila, David Boris, Syed Qadri, Jaime Freitas, Scott Walton, Charles Eddy Jr., Virginia Wheeler (U.S. Naval Research Laboratory)

Atomic layer deposition (ALD) of TiO2 has been widely explored recently due to its promise in non-volatile resistive switch, high-k gate dielectric, solar cell, and photocatalytic applications. This growth method has become increasingly useful as device dimensions are reduced and non-planar complexity is increased. To facilitate epitaxial films at low growth temperatures (Tg), many have investigated plasma, laser or photon, or electron enhanced ALD processes. Specifically for TiO2, it would be beneficial to selectively grow epitaxial anatase or rutile phases in order to tailor optical, catalytic and electrical properties for the required application. Typically, TiO2 phase selectively is attained by varying the underlying substrate, Ti and/or oxidation precursor, or Tg. However, here we demonstrate phase selectivity of high quality epitaxial TiO2 films simply by adjusting plasma gas composition, pressure and Tg.

A Veeco Fiji G2 reactor was used to deposit TiO2 films on different sapphire orientations (c-, m-, a-) with tetrakis(dimethylamido)titanium (TDMAT) and either Ar/O2 or pure O2 plasma at 100-350° C. Previous reports indicate that tuning the ion energy, specifically through substrate biasing, can influence TiO2 film crystallinity and phase [1]. The high pumping speed and large gas flow range available in this specific ALD system provides a wide variation in operating pressures (7-100’s mTorr), which effectively allows tuning of plasma characteristics. Operating at relatively low pressures (9-21mTorr) resulted in a significant flux (0.5-1.5x1019 m-2s-1) of energetic ions (30-50eV), with both the flux and energy decreasing as the pressure is increased. The low pressure conditions yield high-quality epitaxial films at all temperatures, which differs from previous reports using these specific precursors [2,3] likely due to these unique plasma conditions.

Gas composition during the plasma step also had a substantial effect on growth rate, TiO2 phase, and strain. At Tg < 300° C, the growth rate was increased from 0.5 to 0.7 Å /cycle by switching from Ar/O2 to pure O2. Moreover, an O2 plasma produced only rutile TiO2 films, with less strain, independent of growth temperature or underlying substrate orientation. In contrast, films deposited with an Ar/O2 plasma show a phase dependence on temperature and substrate. Films on c-plane Al2O3 go from anatase at Tg below 200° C to rutile above 300° C. The films on m-plane Al2O3 are rutile independent of temperature.

  1. Profijt et al. Electrochem. Sol. Stat. Lett. 15(2) G1 (2012).

  2. Xie et al. J. Electrochem. Soc. 155(9) H688 (2008).

  3. Maeng and Kim. Electrochem. Sol. Stat. Lett. 9(6) G191 (2006).

9:20 AM TF-TuM-5 Substrate Biasing During Plasma Atomic Layer Deposition: From Stress-controlled Oxides to Low-resistivity Nitrides
Harm Knoops (Oxford Instruments, The Netherlands); Tahsin Faraz, Karsten Arts, Saurabh Karwal, Mariadriana Creatore, Erwin Kessels (Eindhoven University of Technology, The Netherlands)

Thin films grown by plasma atomic layer deposition (ALD) can be strongly influenced by the energy of the ions present during the plasma step. Here we report on how using enhanced energies influence the material properties of many oxides and nitrides.1,2 To utilize these effects, discussion on promising process schemes and which material properties are key is needed. This work will discuss application relevant aspects and focus on the stress-control of oxides and the reduction of resistivity for nitrides.

Ion energy control during plasma exposure was carried out in a commercial 200-mm remote plasma ALD system (Oxford Instruments FlexAL) equipped with RF substrate biasing. Substrate biasing increased the refractive index of TiO2 at 300 °C to 2.54±0.03, mass density to 4.2±0.2 g/cm3, and the crystal phase went from anatase to rutile. ALD of TiO2 at 150 °C typically yields amorphous films, but using a bias crystalline rutile phase was obtained. Besides these modified material properties the residual stress was altered from tensile (176±50 MPa) to strongly compressive (-2430±50 MPa) and by choosing the bias voltage and duration, a close to zero stress could be achieved (−25±50 MPa). Stress control of oxides on planar surfaces can be beneficial for several devices through: anti-reflective TiO2 coatings, stressed Al2O3/HfO2 gate oxides, and compressive Ta2O5 barrier layers.

For transition metal nitrides, enhancing ion energies was observed to have pronounced effects on chemical composition and microstructure.1,2 Energetic ion bombardment lowered film resistivity down to 139±10 μΩcm at -187V bias for TiN and increased the crystallinity. The ability to deposit crystalline films on temperature sensitive substrates (e.g., conductive TiN on polymer substrates) could be a benefit for low temperature electronics. Oxygen impurity content was observed to be significantly reduced by substrate biasing. Oxygen contents of 3±2 at. % for TiN and <2 at. % for HfNx were achieved without taking special care of low oxygen backgrounds. Ar/H2 plasma with biasing was found to allow removal of surface oxygen, while simply extending the plasma exposure without applying bias did not lead to such removal. The reduction of oxygen content by these plasmas also suggests a possible use in applications as a chemical sputter clean of surfaces. In general the application of biasing brings the processing of plasma ALD closer to the realm of ALE and atomic scale processing. Possible related processing schemes and the general trends for nitrides and oxides will be discussed.

1Faraz et al., ACS Appl. Mater. Interfaces 10, 13158 (2018)

2Karwal et al., J. Mater. Chem. C6, 3917 (2018)

9:40 AM TF-TuM-6 Development of Novel Superconducting ALD Films for Astronomy Applications
Frank Greer, Peter Day, Byeong-Ho Eom, Henry Leduc (Jet Propulsion Laboratory, California Institute of Technology)

Future sub-millimeter telescopes and spectrometers have the potential to revolutionize our understanding of the formation of the modern universe. Sub-millimeter astronomy can probe the fine structure of the cosmic microwave background, giving glimpses into the early universe immediately following the Big Bang. Recent advances in design have enabled the production of large arrays of cryogenically cooled superconducting detectors with sufficient sensitivity for photon counting applications. Transition edge sensors (TES) and other types of detectors, fabricated from thin films of metal nitrides such TiN, NbN, TaN, VN, and their mixtures, are cryogenically cooled to just below their superconducting transition temperature. Photons in the X-ray or sub-millimeter wavelength range can be detected because their absorption will cause the temperature of an appropriately designed TES array element to rise just enough to cause it to have a finite resistance. However, while conventional deposition processes such as physical sputtering have been sufficient for small area arrays and proof of concept sensors, critical parameters such as the superconducting transition temperature are strongly sensitive to film thickness, stochiometry, and interface quality. Therefore, as the size of an individual detector array or the number of detector arrays on a wafer increases, it becomes more and more difficult to fabricate them with sufficiently uniform response in sufficient quantity to populate the focal plane of a large telescope. We are utilizing the uniformity and compositional control provided by atomic layer deposition to overcome these limitations for the fabrication of transition edge sensors as well as other applications requiring superconducting thin films.

10:00 AM BREAK - Complimentary Coffee in Exhibit Hall
11:00 AM TF-TuM-10 Atomic Layer Deposition of Cobalt Nanoparticles
Gerben van Straaten, Erwin Kessels, Mariadriana Creatore (Eindhoven University of Technology, The Netherlands)

Cobalt nanoparticles (NPs) with well-defined sizes are highly sought after for a variety of catalytic processes. For example, cobalt NP catalysts for the Fischer-Tropsch process, used in industry to upgrade coal and natural gas into fuels, show a sharp maximum in activity for a particle diameter of ca. 8 nm1. Such size-sensitive catalytic processes require conformal deposition of cobalt NPs throughout porous substrates, requirements that can potentially be met by ALD.

ALD of catalytic platinum-group metal NPs with narrow size distributions has been exhaustively studied. However, compared to these systems, ALD of cobalt metal NPs is much more challenging due to their high reactivity. To our knowledge, Co NPs have only been obtained by reduction of ALD-deposited Co3O4 NPs 2 or by reduction and subsequent melting of ALD-deposited Co3O4 layers 3.

In this contribution, we demonstrate for the first time the direct deposition of Co NPs using Plasma-Enhanced ALD (PEALD)4. Cobaltocene (Co(C5H5)2 ) is evaporated at 80oC and dosed for 6s onto an SiO2 surface held at 250oC. Then, exposure to a remote, inductively coupled plasma fed with NH3 (1*10-2 mbar, 100W) takes place for 10s. After each step, 4s of purging takes place, and saturation of all steps was verified with in-situ Spectroscopic Ellipsometry (SE). By means of TEM we find that Co NPs synthesized via this process consist of a metallic core and an oxide shell formed by air exposure, and that their growth takes places in 3 stages. In the initial stage, up to 150 cycles, a homogeneous coverage of small NPs occurs, with an average diameter of less than 1 nm. Then, after 150 cycles, a second population of larger NPs starts to develop, which increase rapidly in size while retaining a narrow size distribution. After ca. 250 cycles this secondary population is fully developed, with a mean diameter of 7.5 nm and a width of 1.7 nm, but the initial population of small NPs remains, yielding a bimodal size distribution. Growth of this secondary population continues up to ca. 550 cycles, at which point we find from in-situ SE that the NPs coalesce into a continuous film. Preliminary GIXRD and XPS analysis reveals that this film consists of crystalline β-Co but that up to 10 at% of C and N can be present in the film.

Experiments are ongoing to test the catalytic activity of Co NPs synthesized via this method towards the Fischer-Tropsch reaction.

1. Den Breejen, J. P. et al. J. Am. Chem. Soc.131, 7197–7203 (2009)

2. Taheri Najafabadi, A. et al. Appl. Catal. A Gen.511, 31–46 (2016)

3. Thissen, N. F. W. et al.Carbon N. Y.121, 389–398 (2017)

4. Lee, H.-B.-R. & Kim, H. Electrochem. Solid-State Lett.9, G323 (2006)

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11:20 AM TF-TuM-11 Atomic Layer Deposition of Ni-Al-O Catalysts for Water Oxidation
Jon Baker, Joel Schneider, Joseph Singh, Adrian Mackus, Stacey Bent (Stanford University)

Electrical energy storage has emerged as an important challenge for societies moving away from fossil fuels toward more renewable sources (e.g. solar and wind) that are intermittent in nature. To rely only on solar and wind for electricity, large scale energy storage systems are required. Among several potential solutions, one promising strategy is to store excess electrical energy in the form of chemical bonds, through electrochemical production of fuels. In particular, electrochemical water splitting enables the production of hydrogen gas as a chemical fuel from excess electrical energy. However, a major drawback of this strategy is the low efficiency of the oxygen evolution reaction (OER).

To improve the efficiency of the OER, understanding why certain catalysts perform well is a powerful tool in designing new catalysts with better properties. Atomic layer deposition (ALD) has emerged as a strong platform to study catalysts due to its ability to deposit films that are compositionally well-defined. For electrochemical systems, ALD has the added benefit of high uniformity, conformality and precise thickness control; these attributes can minimize potential confounding effects like resistive and mass transport losses, which may affect measurement of a catalyst’s intrinsic activity. In this work, we study the effect of aluminum on Ni-Fe-OOH catalysts. While Ni-Fe-OOH catalysts have been reported to have high activity for the OER (nearly independent of synthesis method), the impact of aluminum on this catalyst is not yet well understood, with a wide range of OER activities reported. By using ALD, ternary films of Ni-Al-O were deposited. Introduction of iron was achieved through electrolyte iron-doping, enabling the formation of the quaternary Ni-Al-Fe-OOH catalyst. Electrochemical characterization of sufficiently thin films found that aluminum improved the turnover frequency of Ni-Fe-OOH catalysts by a factor of 3-4. By carefully controlling composition of the catalyst, the effects of aluminum and iron were independently studied. In iron-free catalysts, aluminum was found to improve OER activity; however, its improvement in activity was small compared to the effect of iron. In addition, thickness dependent studies were performed to understand how morphology and synthesis method may impact observed activity. For Ni-Al-Fe-OOH catalysts, a strong thickness dependence in the OER activity was observed. Characterization of the catalysts ex situ and under OER operating conditions reveal the likely cause of this thickness dependence is the observed changing morphology of the film with increasing thicknesses.

11:40 AM TF-TuM-12 Atomic Layer Deposition of Bismuth Vanadate Photoanodes
Sudarat Lee, Ashley Bielinski, Samuel Esarey, James Brancho, Bart Bartlett, Neil P. Dasgupta (University of Michigan, Ann Arbor)

Bismuth vanadate (BVO) has been widely studied as one of the most promising photoanode materials for photoelectrochemical (PEC) water splitting, owing to a bandgap of 2.4 eV and favorable band positions for water oxidation. However, while BVO has the potential for high anodic photocurrents, it is often limited by electron-hole separation, charge transport, and water oxidation kinetics, requiring development of nanostructured electrode architectures to optimize performance.

Here we demonstrate a new method for fabricating tunable BVO photoanodes deposited by atomic layer deposition (ALD). We present the first deposition of ALD BVO using Bi(OCMe2iPr)3 as the bismuth source, vanadium(V)oxytriisopropoxide as the vanadium source, and water as the oxidant. The choice of this recently developed Bi precursor provides full control of Bi:V stoichiometry in contrast to the use of earlier Bi precursors such as triphenylbismuth1, while also providing a per cycle growth rate that is more than three times higher than previous reports.

The BVO films were deposited as a nanolaminate of binary bismuth and vanadium oxides. The films were post-annealed to achieve the photoactive monoclinic BiVO4 phase. Film composition and photocurrent were investigated as a function of deposition pulse ratio and film thickness. The photoactivity of planar ALD BVO was measured in a three-electrode cell under simulated AM1.5G illumination. Sulfite oxidation was used to optimize the BVO deposition conditions independent of co-catalyst performance.We achieved the highest reported photocurrent to date for ALD photoanodes. Using a planar electrode with a 40nm thick BVO film, a photocurrent of >2.6 mA cm-2 at 1.23 VRHE was demonstrated for sulfite oxidation and a photocurrent of >1.1 mA cm-2 at 1.23 VRHE was demonstrated for water oxidation using an un-optimized cobalt co-catalyst.

ALD provides conformal coverage of high aspect ratio structures. The development of an ALD process for BVO enables core-shell architectures that help address the charge transport and carrier separation challenges by decoupling carrier diffusion and light absorption lengths. To demonstrate this benefit, BVO was deposited on mesoporous tin oxide (ITO) substrates to form 3-D electrode architectures with tunable absorption and charge extraction properties. The photoresponse was enhanced for both sulfite and water oxidation under illumination, demonstrating the power of ALD to improve light absorption and charge extraction in 3-D nanostructured electrode architectures.

(1)Stefik, M. Atomic Layer Deposition of Bismuth Vanadates for Solar Energy Materials. ChemSusChem

2016, 9 (13), 1727- 1735.

12:00 PM TF-TuM-13 ALD of Cobalt Phosphate Electro-catalyst for Oxygen Evolution Reaction
Valerio Di Palma (Eindhoven University of Technology, The Netherlands); Georgios Zafeiropoulos, Michail Tsampas (DIFFER); Wilhelmus Kessels, Mariadriana Creatore (Eindhoven University of Technology, The Netherlands)
Sunlight-driven water splitting is widely investigated through studies addressing both the development of earth-abundant element-based (photo)electrodes, as well as engineering of compact, efficient and inexpensive devices. One important reason that keeps these systems from being of practical use to date is the sluggish kinetics (high overpotential) of the oxygen evolution reaction (OER). OER is a four electron-proton coupled reaction, while hydrogen evolution reaction (HER) is only a two electron-transfer reaction, and hence usually OER requires a higher overpotential [1].

Electro-deposited cobalt phosphate (CoPi) has been reported as a valid OER catalyst (as bare electrode or as cocatalyst in photoelectrodes) and alternative to rare element compounds [2]. In parallel, ALD is rapidly reaching out also to (photo-)electrocatalytic applications [3,4] and in the present contribution we investigate CoPi growth by ALD and its application as OER catalyst. Compared to electrodeposition, ALD offers the major advantage of tuning the electro-catalyst chemical composition, as we demonstrate in this work.

The CoPi process has been developed using an ABCD scheme, where A, B, C and D are the cobaltocene (CoCp), first oxygen plasma (5 s), trimethylphosphate (TMP) and second oxygen plasma (2 s) dosing steps, respectively. The deposition performed at 300oC results in a growth per cycle of about 1.1 Å, determined by in-situ spectroscopic ellipsometry. Elemental characterization by XPS of the ALD prepared CoPi shows a stoichiometry of Co3(PO4.3)2. The film deposited on FTO/glass is active for OER, as confirmed by cyclic voltammetry. The current density peak measured is about 1.0 mA/cm2 at 1.4 V vs. NHE (Normal Hydrogen Electrode), comparable to the values reported in literature for electrodeposited CoPi. Furthermore, we investigated the effect of Co-to-P concentration ratio on the OER catalytic activity. To this purpose, the process was modified from the aforementioned ABCD approach into an (AB)x(CD)y supercycle approach. The sample prepared with a ratio x-to-y of 12-to-11 showed already a higher current density peak (about 1.2 mA/cm2 at 1.4 V vs NHE) than the sample deposited without the use of supercycles. This finding highlights the role of Co-to-P concentration ratio on the activity of the catalyst and how the ALD supercycle approach can be adopted to this purpose.

References

[1] J. Surendranath et al., J. Am. Chem. Soc., 132 (39) (2010) 13692

[2] M.W. Kanan and D.G. Nocera, Science, 321 (2008) 1072

[3] A. Paracchino et al., Energy Environ. Sci., 5 (2012) 8673

[4] S.C. Riha et al., ACS Nano, 7 (3) (2013) 2396

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Session Abstract Book
(322KB, May 6, 2020)
Time Period TuM Sessions | Abstract Timeline | Topic TF Sessions | Time Periods | Topics | AVS2018 Schedule